This invention relates to a superconducting thick film circuit board, a process for producing the same, a thick film superconductor, and a process for producing the same.
Intermetallic compounds of Nb.sub.3 Sn, GaV.sub.3, Nb.sub.3 Ge, etc. used as a superconductor heretofor are as high as 16.8 to 23K in the critical temperature compared with known NbTi and show a critical magnetic field of 20 to 40 teslas at 4.2K, so that they were practically used as superconducting coils, Josephson devices, etc. But these compounds of Nb.sub.3 Sn, GaV.sub.3, Nb.sub.3 Ge, etc. have a defect in that the temperature for showing superconductivity is lower than the liquid nitrogen temperature (77.3K). For example, the temperature showing superconductivity (hereinafter referred to as "T.sub.c.sup.onset ") and the temperature showing complete superconductivity and resistivity of zero (hereinafter referred to as "T.sub.c.sup.zero ") are 30K or less, respectively. Therefore, in order to use these compounds as a superconductor practically, it is necessary to use very expensive liquid helium, which disadvantageously requires complicated apparatus.
In order to improve such defects, super-conductors using compounds of Ba-La-Cu-O series were developed as disclosed in New Superconducting Materials Forum, 1st Symposium Proceeding, pp. 24-33 (1987). By these superconductors, the critical temperature was raised above 30K. Further, by superconductors of Y-Ba-Cu-O series compounds subsequently found, the critical temperature was raised to the order of 90K, which temperature is higher than the liquid nitrogen temperature of 77.3K. A superconductor using a Y-Ba-Cu-O series compound has a critical current density (hereinafter referred to as "Jc") of as high as 1.8.times.10.sup.6 A/cm.sup.2 at the liquid nitrogen temperature when measured as to a single crystal thin film formed by sputtering on a strontium titanate (SrTiO.sub.3) single crystal substrate (New Superconducting Material Research Society, 3rd Symposium Proceeding, pp. 61-75 (1987)).
On the other hand, a superconducting thick film circuit board obtained by adding Ag.sub.2 O to superconducting powder of Y-Ba-Cu-O series has Jc of 1300 A/cm.sup.2 at the liquid nitrogen temperature as disclosed in a Resume of Lectures at Autanum Meeting of the Society of Powder Metallurgy, p. 24 (1988).
But in the former case, although there is obtained a high Jc of 1.8.times.10.sup.6 A/cm.sup.2, there are many defects in that the cost is very high due to the use of a single crystal substrate, the size of substrate is limited, and it is difficult to connect circuits formed on both sides of the substrate via a through-hole.
On the other hand, in the latter case, the Jc value of 1300 A/cm.sup.2 is the case of the circuits having 40 .mu.m in thickness and 5 mm in width, but Jc is lowered to 600 A/cm.sup.2 or less when the circuits have a thickness of 10 .mu.m and a width of 1 mm as shown in "Nikkei Super-conductors" vol. 18, p. 2 (published on Oct. 3, 1988). Therefore, when a superconducting thick film circuit board having circuits of 0.5 mm or less in width is made, it can only be obtained the Jc value of as low as 600 A/cm.sup.2 or less.
Further, a thick film superconductor of Y-Ba-Cu-O series has a defect in that Jc is greatly lowered by an external magnetic field as shown in Japanese Journal of Applied Physics, vol. 27, No. 2, pp. 185-187 (1988).